The invention relates to a process for producing one or more electrical conductors on a photovoltaic device, this process being particularly suitable for producing collecting conductors on a photovoltaic cell, and to a process for manufacturing a photovoltaic cell incorporating such a process. The invention also relates to a unit for producing photovoltaic cells implementing such a process and to a photovoltaic cell per se obtained by such a process.
A photovoltaic cell is manufactured using a wafer made of a semiconductor, generally silicon. This manufacture in particular requires electrical conductors to be formed on the surface of the wafer. FIG. 1 illustrates the surface of such a wafer 1 according to the prior art, which comprises first parallel conductors of thin width, called collecting conductors 2 or collecting fingers, the function of which is to collect the electrons created in the silicon by light. In addition, the surface of the wafer 1 comprises other wider parallel conductors 3, generally called buses, oriented in a direction perpendicular to the collecting conductors 2, the function of which buses is to carry larger currents from photovoltaic cell to photovoltaic cell. All of these conductors 2, 3 are obtained by different techniques allowing continuous conductive lines to be formed, which lines extend continuously across the entire length and width of the wafer.
To produce these conductors, one prior-art method consists, for example, in screen-printing a conductive ink on the wafer, in one or two screen-printing steps. Such a process does not allow an ideal geometry to be obtained, especially as regards the uniformity of the height of the layer of ink deposited, and does not allow conductors with a sufficiently satisfactory performance to be formed. Specifically, the electrical performance of these conductors is very sensitive to their geometry, and especially to their thickness/width ratio, thickness being measured in the vertical direction perpendicular to the wafer 1, and width being measured in the horizontal direction, transverse to the conductor.
As a variant, document EP 0 729 189 suggests using another printing technique to produce these conductors, which technique consists in replacing the masking fabric used in screen-printing with a metal stencil in which through-apertures are produced. However, in order not to weaken these metal masks, and to obtain an optimized behaviour during printing, the area of the apertures is limited and the process requires at least two printed layers to be superposed using two separate, complementary masks. For this reason, this process remains complex and expensive.
Furthermore, the widest conductors 3 are in general covered with a metal strip, made of copper, the strip being soldered over its entire length, using a solder-covered copper strip. This strip extends across the entire length of the cell and serves to connect cells to one another, the strips of these cells being soldered in order to electrically connect a plurality of cells in order to form a photovoltaic module.
Such a copper strip is fastened to a photovoltaic cell by placing the strip on the conductor 3, then by bringing various soldering heads to bear, the transmitted heat of which, generally transmitted by infrared or hot air, allows the copper strip to be soldered over its entire length, the thermal conductivity of the strip promoting the transmission of the heat over its entire length, and therefore the continuous production of the solder joint. Thus, a copper strip soldered over the entire length of the conductor 3 is obtained on the front side of the photovoltaic cell.
Interconnection of a plurality of cells, in order to form a module, is also achieved by soldering the various copper strips. The temperature increase during these soldering steps and the different expansion coefficients of the various materials of a photovoltaic cell means that there is a risk of damage to the structure of the photovoltaic cells: specifically, sometimes micro-cracks appear in the structure of the silicon. In addition, the photovoltaic cells are also subjected to stress due to temperature variations during their ordinary use, which adds a risk of the cell deteriorating because of expansion effects. These risks increase with the thickness of the copper strip used and may lead to significant decreases in the performance of a photovoltaic device. Moreover, at the present time, cells are increasingly thinner and therefore increasingly sensitive to stresses, and the copper strips used are increasingly thicker in order to respond to the increase in current due to improvement in the conversion efficiency of these cells.